This invention relates to non-destructive testing and, more particularly, to a method and apparatus for nondestructive inspection of a solid workpiece using ultrasonic energy.
Ultrasonic energy has been used for many years for non-destructive testing of various types of materials. In general, ultrasonic energy is injected into a workpiece to be tested or inspected, and the ultrasonic energy transmitted through or reflected from the workpiece reveals characteristics of the structure of the workpiece. The ultrasonic energy received from the workpiece can be imaged or presented in other ways.
Non-destructive testing frequently uses ultrasound transmission in solids in a shear mode since the velocity of propagation is lower than in the compressional mode and, consequently, the resolution is better. A fluid (such as water) is typically used to couple the ultrasound energy between the transmitting and/or receiving transducer and the solid workpiece to be inspected. Fluids do not support transmission of shear mode waves. Accordingly, a compression mode wave in the fluid is converted to shear mode by inclining the incident beam with respect to the surface of the workpiece.
In steel, for example, conversion of compression mode to primarily shear mode occurs at an angle of approximately 15 degrees with respect to the normal to the workpiece surface. (This results in an angle of beam energy in the steel of about 60 degrees with respect to the normal). This is typically achieved with a small aperture transducer inclined at an angle of about 15 degrees with respect to the normal to the surface. (It can be noted that at angles greater than about 22 degrees, most of the energy is reflected.)
It is among the objects of the present invention to provide a technique which allows inspection in solids such as steel with larger aperture transducers while obtaining the desired shear mode transmission in the workpiece. Among the further objects of the invention are to improve the ultrasonic inspection of solid workpieces with ultrasonic energy in the shear mode, and to provide a technique for inspecting with both shear and compressional mode ultrasonic energy.